The present invention concerns instruments which measure the conduction angle of ignition coils in battery-powered ignition systems for internal combustion engines, especially for example vehicular engines. Instruments of the type here in question typically comprise means for deriving voltages from the ignition system itself and include evaluating circuitry or converting such derived voltages into an indication or display of the conduction angle of the ignition coil.
Federal Republic of Germany published patent application DE-OS 24 43 402 discloses an arrangement for measuring the conduction angle of an ignition-system ignition coil, the input circuitry of the electrical instrument comprising a protective circuit and connected to the output of the latter an operational amplifier operative through the intermediary of a filter for controlling a second operational amplifier, which latter in turn actually generates a signal providing the desired indication of coil conduction angle. One input of the first operational amplifier is connected to a voltage divider and maintained at a constant level. Also, both inputs of the operational amplifier are protected against excessively high voltage by means of protective diodes. This known instrument provides an exact measurement of conduction angle. However, when this measurement is to be from time to time performed on ignition systems of various differing conventional types--e.g., elementary ignition systems in which interrupter contacts open to directly interrupt ignition-coil current; germanium or silicon transistor electronic ignition systems; regulated current transistorized ignition systems; capacitor ignition systems incorporating silicon-controlled rectifiers or switches--, it is necessary to switch over the input of the operational amplifier from one voltage level to another, in order that the measurement of coil conduction angle be properly performed for the particular type of conventional ignition system involved. These switchovers of voltage level are performed manually by the technician using the instrument.